The relationships among renal substrate uptake, the metabolic fates of substrates and the several renal functions in both the dog kidney in vivo and in the isolated perfused rat kidney are being studied. Since more substrate is taken up by kidney than can be oxidized, we are attempting to determine whether only certain substrates are oxidized to provide energy for renal work functions (particularly, Na ion reabsorption). The other substrates taken up by the kidney are utilized in such a fashion as to result in substrate conservation by being converted to more readily utilized or stored products than were the original substrates. The hypothesis that the products formed by the substrate interconversion mechanism can be regulated by ECF-pH is being tested.